1. Field of the Invention
This invention relates to low iron loss grain oriented silicon steel sheets and a method of producing the same, and more particularly to grain oriented silicon steel sheets having an iron loss considerably reduced by locally pushing a surface layer of the steel sheet into a base metal to conduct refinement of magnetic domains.
2. Related Art Statement
The grain oriented silicon steel sheets are manufactured through complicated and many steps requiring severe controls, wherein secondary recrystallized grains are highly aligned in Goss orientation, and a forsterite layer is formed on a surface of base metal for steel sheet and further an insulative layer having a small thermal expansion coefficient is formed thereon.
Such a grain oriented silicon steel sheet is mainly used as a core for transformer and other electrical machinery and equipment. In this case, it is required that the magnetic flux density (represented by B.sub.10 value) is high and the iron loss (represented by W.sub.17/50 value) is low as magnetic properties, and the insulative layer having good surface properties is provided.
Particularly, supreme demands on the reduction of power loss become conspicuous in view of energysaving, so that the necessity of grain oriented silicon steel sheets having a lower iron loss as a core for the transformer becomes more important.
It is no exaggeration to say that the history of reducing the iron loss of the grain oriented silicon steel sheet is a history of improving secondary recrystallization structure of Goss orientation. As a method of controlling such a secondary recrystallized grain, there is practiced a method of preferentially growing the secondary recrystallized grains of Goss orientation by using an agent for controlling growth of primary crystallized grain such as AlN, MnS, MnSe or the like, or a so-called inhibitor.
On the other hand, different from the above method of controlling the secondary recrystallization structure, there are proposed epock-making methods, wherein local microstrains are introduced by irradiating laser onto a steel sheet surface (see T. Ichiyama: Tetsu To Hagane, 69(1983), p895, Japanese Patent Application Publication No. 57-2252, No. 57-53419, No. 58-24605 and No. 58-24606) or by plasma irradiation (see Japanese Patent laid open No. 62-96617, No. 62-151511, No. 62-151516 and No. 62-151517) to refine magnetic domains to thereby reduce the iron loss. In the steel sheets obtained by these methods, however, the microstrain is disappeared through the heating upto a high temperature region, so that these sheets can not be used as a material for wound-core type transformers which are subjected to strain relief annealing at high temperature.
Furthermore, there is proposed a method of causing no degradation of iron loss property even when being subjected to strain relief annealing at high temperature. For example, there are a method of forming groove or serration on a surface of a finish annealed sheet (see Japanese Patent Application Publication No. 50-35679 and Japanese Patent laid open No. 59-28525 and No. 59-197520), a method of producing fine regions of recrystallized grains on the surface of the finish annealed sheet (see Japanese Patent laid open No. 56-130454), a method of forming different thickness regions or deficient regions in the forsterite layer (see Japanese Patent laid open No. 60-92479, No. 60-92480, No. 60-92481 and No. 60-258479), a method of forming different composition regions in the base metal, forsterite layer or tension insulative layer (Japanese Patent laid open No. 60-103124 and No. 60-103182), and the like.
In these methods, however, the steps become complicated, and the effect of reducing the iron loss is less, and the production cost is high, so that such methods are not yet adopted industrially.